Read accelerometer from raspberry pi

Hello there :)

I am facing problems with reading values from a Bosch BMA280 Accelerometer connected to a raspberry pi via Matlab.

What is working so far:

• Raspberry Pi Support Package is installed
• Raspi is addressable via Matlab
• BMA280 Sensor is connected to Raspberry Pi via SPI 4-wire
• I can read several register via writeRead-Function (SPI) in Matlab (for example Chip ID, X-, Y-, Z-Data)

What I want to do:

• Use the BMA280 connected to the Raspi in Simulink

My problem is, I don't know where to start. I found device drivers on the bosch homepage in c-Code. Should I try to build my own device driver in matlab? Are there any better ways?

The other problem I am facing: The bosch sensor sends the X-, Y- and Z-Data in two's complement format. The matlab function "writeRead" (Raspberry support package) returns a row vector, which looks like this:

The technical documentation tells the user to first read the LSB-Register (here 0x04, which has to be 0x84 for reading values) and afterwards read the MSB-Register (0x05 -> 0x85). I don't know why I, but when I'm not including "hex2dec('0')", matlab reads only one register. Can anyone explain that to me? This would look like (0x84 should not be "0"!):

How do I convert the row vector returned by matlab to useful values?

Thanks so far.

Kind regards,

Niko

[Joachim Schlosser]

You are doing good. What else would you need than reading and writing registers?

You probably found this already – The Raspberry Pi SPI doc for MATLAB is in the Online Doc on Raspberry Pi SPI in MATLAB, with a page on General Usage and the Function Reference.

[Student88]

Thanks for your reply :) I guess reading and writing the registers is the main deal. I am just unsure about the device driver from bosch, which is in c-code.

My idea now is the following: I found a way to develop device drivers for simulink: Simulink device driver

This one has a good documentation, so I am hoping to get a functional simulink block for reading the bma280 sensor. I will try to use the wiringpi library for the spi connection on the raspberry pi (as there is no spi block available in simulink).

---- new approach:

While reading through the documentation of "device drivers for simulink", I found out, that there are different ways to write a device driver for simulink:

• S-Function (via S-Function Builder Block)
• Legacy Code Tool
• Matlab Function
• System Object

As I need to be able to change the sample time of the device driver and would like to achieve a high reading frequency of the bma280 sensor (about 2000 Hz), I guess matlab function is the best way to start.

---- end of new approach :)

The next step would be to implement the given functions from the bosch device driver into that simulink device driver.

If you have any further suggestions, let me know :)

Thanks for the link by the way (and yes, I already found that:)) The online doc for the writeRead function tells the user to:

"To read data from SPI, send dummy values". Is that the reason, why I have to send hex2dec('0') when I only want to read some registers (see first picture above)?

[Joachim Schlosser]

Yes, if you want to write the code yourself, go MATLAB Function. If you just want to re-use existing C functions, use Legacy Code tool.

On writeread: yes, this is how the writeread function seems to be proposed by the SPI standard.

[Student88]

Hello there :)

Some news and new questions from me :D

I got the BMA280 Sensor to work with the Raspberry Pi (C Code). It returns correct acceleration values (in G).

Next step would be to implement this c code in a matlab function block. I've tested this with a LED before (running the code on the raspberry pi), and it worked.

But... It doesn't work with my c code (which is working perfec on the raspberry though).

First problem: Printf does not work the way I would expect it to work (printf command in c-code). At least Matlab only prints out the messages, when a printf-command in the matlab script is called. I found solutions on the internet using "ssprintf", but when I'm including "simstruc.h" in my c-code (which is needed for ssprintf), I'm getting errors saying:

In file included from ./SPI_Raspi.h:5:0, from ./SPI_Raspi.c:3: ./simstruc.h:1076:5: error: unknown type name ÔÇÿmxArrayÔÇÖ ./simstruc.h:1879:6: error: unknown type name ÔÇÿmxArrayÔÇÖ ./simstruc.h:1883:3: error: unknown type name ÔÇÿ_ResolveVarFcnÔÇÖ In file included from ./SPI_Raspi.h:5:0, from ./SPI_Raspi.c:3: ./simstruc.h:11996:5: error: unknown type name ÔÇÿmxArrayÔÇÖ _bma280_device_driver.mk:125: recipe for target 'linuxUDP.o' failed make: * [linuxUDP.o] Error 1

Is there any workaround or other way, to solve this?

My second problem is the following: The matlab code, using my c-code, looks like this:

    % code
function [x, y, z]=BMA280()
persistent initflag;
x=0.0;
y=0.0;
z=0.0;
temp = 0;

if strcmp(coder.target, 'rtw'),

    if isempty(initflag),
        % Initialize step
        temp = coder.ceval('bma280Init');
        if (temp), 
            initflag = 1;
        else
            fprintf('Failed to initialise BMA280!\n');
            % End function because of error
            return;
        end
    else
        % output Step
        coder.ceval('updateAccelData', coder.wref(x), coder.ref(y), coder.ref(z));
        %fprintf('X is %10.5f    Y is %10.5f     Z is %10.5f \n', x, y, z);
    end

end

(sorry about formatting, I don't know how to insert code the right way here)

I guess the problem has got something to do with the arguments for the function "updateAccelData". This c-function needs pointers to float values, which contain the x, y and z accel data after calling the function. I found out, that I need to use "coder.ref" to pass the reference (pointer) to my c-function. But it does not work (as you can see, I also tried "coder.wref", which only offers write access, but that did not change anything). Is there a problem with the data type "float" in c, regarding matlab? By the way, the c-Function prints the correct values, when I am running my modell in simulink on external mode (raspberry). But Matlab does not contain anything in the values x, y and z.

The last question from my side would be: is there any way to debug matlab code, which uses my c-code, and runs on the raspberry pi? I couldn't find a solution until now.

Have a nice weekend,

best regards,

niko